The Benguela upwelling ecosystem lies adjacent to the south-western coast of Africa, from southern Angola

south-western coast of Africa, from southern Angola (15°S) to Cape Agulhas (35°S; Fig. 1). Ecologically, it is split into separate northern and southern subsystems by a zone of intense perennial upwelling near Lüderitz (26–27.5°S; Shannon 1985). As is characteristic of upwelling ecosystems, the Benguela is highly productive and supports abundant populations of plankton-feeding clupeoids, including anchovy Engraulis capensis, round herring Etrumeus spp. and sardine Sardinops sagax, all of which are commercially exploited (Armstrong and Thomas 1989). Sardine are distributed from southern Angola to KwaZulu-Natal on the north-east coast of South Africa, and despite wide-ranging migrations, there appear to be two separate stocks. The northern stock extends along the Namibian coast from the Lüderitz upwelling cell to the warm-water front off southern Angola (~15°S), and the southern stock is found from the Orange River to Kwazulu-Natal (27°S; Beckley and van der Lingen 1999). Tagging studies indicated no movement of sardine from the Western Cape to Namibia and only minimal movement in the opposite direction (Newman 1970), and genetic studies have shown no differences between the northern and southern stocks (Grant 1985). The Benguela sardine is a population of the circumglobally distributed Sardinops sagax, additional populations being found in some of the other upwelling ecosystems of the world (Parrish et al. 1989, Grant and Leslie 1996). Sardine have formed the basis of important fisheries in both South Africa and Namibia since the late 1940s (Crawford et al. 1987). Catches in both countries were high during the 1950s and early 1960s, but declined rapidly thereafter and have remained relatively low since then (Beckley and van der Lingen 1999). Off South Africa, hydroacoustic estimates of sardine biomass have shown that the stock size has grown steadily from the mid 1980s to the present (Barange et al. 1999). Off Namibia, signs of a population recovery in the early 1990s were followed by a decline in biomass that rendered the northern sardine population virtually commercially extinct by the end of 1995 (Boyer et al. 1997). Since then, the population has shown a slight increase, but it is still below the levels of the early 1990s (Fig. 2). Large fluctuations in sardine biomass have been described in all regions of the world where the species is intensively fished (Lluch-Belda et al. 1989, Schwartzlose et al. 1999). Although variability in population size is commonly attributed to conditions affecting early life-history stages and hence recruitment vari123